The class of particle separation techniques known as field flow fractionation, or FFF, has become increasingly popular in recent years. This is evident from an examination of the extensive and detailed bibliography to be found on the World Wide Web at http://www.rohmhaas.con/fff/fff.html. These FFF techniques consist of constraining a sample bearing fluid to flow within a long thin channel by means of an applied pressure gradient along its long dimension. The channel is often comprised of upper and lower flat plates separated by means of a spacing element, of thickness much smaller than the channel width, which also seals the channel and defines its horizontal dimension. In response to the pressure gradient, the fluid moves and its velocity assumes the well known quadratic Pouiselle profile. The fluid touching the plates and spacer is stationary and its velocity reaches a maximum in the center of the channel. A field is then applied perpendicular to the direction of flow. The resulting force on the particles causes them to migrate towards one or both of the plate surfaces, depending on the sign of the force. The magnitude and size dependence of the force depends on the nature of the applied field, but in all of these techniques, the particle's concentration profile will be due to a balance between the applied force which tends to concentrate the particles near the surface, and effects of diffusion which tend to reduce the concentration. The assumption here is that the local concentration is small enough that the particles do not interact. If the field is too large, the particles will be forced onto the surface. This is known as steric mode separation and will not be discussed further in this specification.
For any specific particle population, the mean distance from the plate surface can vary quite dramatically, depending on the particle size, shape, and the magnitude of the field coupling. Since there is a strong velocity shear near the surface of the plates, the particles with larger mean displacements will be in the faster moving part of the fluid stream and will be transported more rapidly along the channel. Therefore they will elute earlier than those closer to the surface.
Many different applied fields have been used, each of which separates the particles by a different mechanism. The most frequently used fields have been i) gravitational which is usually referred to as sedimentation, produced by rotating the channel in a centrifuge; ii) thermal, produced by a temperature difference between two isothermal plates; iii) hydrodynamic generally called flow, produced by flow through a semipermeable membrane; and iv) electrical, produced by conducting plates which act as electrodes. It is this latter FFF process to which this specification is primarily directed.
There has long been a need to find better electrode means for electrical FFF separation devices. The traditional electrical FFF channel is comprised of two solid conducting bars of metal or graphite, which are separated by a thin spacer which has a channel cut from it. Typically, the spacer is made of made from MYLAR.RTM. or similar material which is slightly deformable under an applied clamping pressure. The spacer thus serves to define the lateral extent of the cell, provides the fluid seal, and electrically insulates the plates from each other. The fluid is introduced into the cell through holes at the ends of the conducting plates. Since the separation efficiency depends on the length of the channel and the strength of the field, there is a strong incentive to make the cells long and to make the spacing between the electrodes as small as possible. Moreover, as the field strength is increased, the particle mean distance from the electrode surface decreases. It it not uncommon for this distance to be a few micrometers. There is, therefore, a strong incentive to make the electrode surfaces optically smooth. In addition to minimizing surface roughness, there is a need to maintain the plates spacing over the entire length of the channel since the homogeneity of the field is essential in achieving consistent and reproducible separations while, at the same time, minimizing zone broadening and sample remixing. Finally there is a need to improve the hardness and stability of the plates. As with all FFF devices, it has always been useful to visualize the separation process, but since the plates are generally opaque, this feature is rarely achieved.
Many of the materials used in extant electrical FFF separators are chemically or mechanically unstable. For example, graphite electrodes, no matter how well prepared, tend to shed particles and deteriorate with time. Electrodes made of titanium often oxidize under exposure to some of the mobile phases frequently used in the device. Platinum coated electrodes, traditionally made from a very thin plating onto steel plates, cannot withstand the typical electrical fields produced and tend to fracture exposing the reactive steel supporting structures. Plates made entirely of platinum would be ideal but because of their prohibitive cost are never used.
A new concept and design for the electrical FFF structure has been developed and is described herein. The new plate structure incorporates all of the most desirable features for electrical FFF electrodes and provide means for visualizing the flow as well.